1. Field of the Invention
The present invention relates in general to a printing apparatus that prints full-color images by transferring overlapped different color images by use of a plurality of electrostatic recording units equipped with the function of electrophotographic recording printing and, more particularly, to a printing apparatus that corrects a wow and flutter of color images caused by a flutter in the speed of a plurality of detachable electrostatic recording unit's photosensitive drums or a wow and flutter of a drive system including belts which convey recording paper.
2. Description of the Related Art
The conventional color printing apparatus utilizing electrophotographic recording places in tandem the four-color (black (K), cyan (C), magenta (M), and yellow (Y)) electrostatic recording units in a direction in which recording paper is conveyed. Those four-color electrostatic recording units form latent images by optically scanning a photosensitive drum based on the image data and develop the latent images with color toner and then overlap and transfer those latent images onto constantly moving recording paper in order of yellow (Y), magenta (M), cyan (C), and black (K) and finally heat and fix them with a fixing apparatus. The yellow (Y), magenta (M), cyan (C), and black (K) electrostatic recording units must be replaced totally or partially immediately after the color toner has run out. For this purpose, the electrostatic recording units have a structure such that they can easily be detached with the unit cover as opened.
The color printing apparatus in which Y, M, C, and K electrostatic recording units are placed in tandem in a direction of conveying recording paper must improve the accuracy of color matching by decreasing the wow and flutter of toner images transferred onto moving recording paper by those electrostatic recording units. If, for example, the resolution in both the main scanning direction (perpendicular to the conveying direction) and the sub-scanning direction (conveying direction) of recording paper is 600 dpi, the pixel pitch is about 42 micrometers, so that the wow and flutter must be decreased to 42 micrometers or less. Those color printing apparatus of a structure that Y, M, C, and K electrostatic recording units are placed in tandem in a recording-paper conveying direction transfer toner images by rotating the Y, M, C, and K electrostatic recording units at a constant speed while conveying a conveyer belt which sucks the recording paper. With this, to increase the resolution in a sub-scanning direction, i.e. recording-paper conveying direction, the circumferential rotational speed of the belt and the circumferential speed of the photosensitive drum must be held at a constant value. The belt conveyer system transmits motor rotations via a gear train to a belt pulley, to circulate the belt. Likewise, the photosensitive drum drive system transmits motor rotations via a gear train to the photosensitive drums. Those systems, however, are susceptible to an axial eccentricity due to a mechanical inaccuracy of the rotational axis, which eccentricity causes a periodical flutter in the belt's circumferential rotational seed and circumferential speed. Therefore, even if the belt drive motor and the drum drive motor are accurately controlled at a prescribed target speed, an inevitable wow and flutter develops in the belt's circumferential rotational speed and circumferential speed caused by an eccentricity of the rotational center, thus fluctuating the pixel pitch. To decrease the wow and flutter of pixels because of a speed flutter, the mechanical processing accuracy and assembly accuracy for the rotating portions of the drive systems must be improved, thus greatly increasing the manufacturing cost. For example, it has been difficult to control a speed flutter mechanically so as to decrease the printing error down to 42 micrometers or less within a practical cost. With this, the conventional tandem-type color printing apparatus bring about as large as a 300 micrometers of a wow and flutter, being difficult to provide a practical printing quality by use of overlapping of color components.